`Spontaneously' emerging patterned neuronal activity during early development prior to the onset of sensory experience is a widely observed phenomenon in vertebrate nervous systems. Since the first discovery of the waveform of spontaneous activity in the developing rodent retina over 20 years ago, multiple instances of this activity have been demonstrated to play an important role for the refinement of neuronal architecture. However, despite a history of research, our understanding of specific mechanisms such as how this activity appears, propagates, or shapes development is far from complete, mainly due to the technical challenges of manipulating the phenomenon in intact animals. Examining similar processes in a model system with improved genetic tools, such as in Drosophila would provide greater experimental access for understanding this process. However, it remains unclear whether such robust spontaneous neuronal activity that shapes neurodevelopment in vertebrates exists in simpler invertebrate model systems. In preliminary calcium imaging experiments in Drosophila, I observed the existence of robust patterned spontaneous calcium activity in photoreceptor neurons during early retinal development before sensory processing begins. This activity is reminiscent of mammalian retinal waves, thus I propose three aims designed to determine the role and underlying molecular mechanisms of this activity in the developing Drosophila visual system. In aim 1, I will combine genetically encoded calcium and voltage indicators with cell-type specific Gal4 driver lines to fully characterize the cell types engaged in this activity throughout the developing visual system. In aim 2, I will determine the cellular and molecular mechanisms that generate this spontaneous activity using two approaches. First, I will establish whether widely used genetic tools for manipulating neuronal activity in adult animals, such as inward rectifying potassium channels (Kir2.1), can influence spontaneous activity during development. Second, I will conduct a candidate RNAi screen for genes that are required for generating patterned spontaneous calcium activity. Finally, in aim 3, as instructed by mammalian studies, I hypothesize that patterned spontaneous activity plays a functional role in refining neuronal wiring of photoreceptor neurons and will determine which specific developmental processes are regulated by this activity using the mechanistic understanding gained from aim 2. This work aims to establish an invertebrate model of patterned activity-dependent neurodevelopment. This will greatly facilitate mechanistic and detailed in vivo studies of this poorly understood developmental phenomenon and make use of the powerful genetics and genome-wide screening capability available in Drosophila. Ultimately, this will promote an understanding of the molecular mechanisms of activity-dependent neural plasticity, which has critical roles in a variety of neurodevelopmental diseases, including autism spectrum disorder.